The Last Word

Question: For Christmas I was given a device for warming bottles of red wine. It is a plastic jacket filled with transparent gel which wraps around the bottle. When you press a stud on the jacket the gel gradually becomes opaque and warm. The opacity spreads out from the stud, taking about 30 seconds to cover the gel. To regenerate the jacket you immerse it in boiling water for 15 minutes until the gel is transparent again. How does it work?

Answer: The jacket of your wine heater contains a solution of sodium acetate trihydrate in water. This solution has two unusual properties that make it ideal for use in these "heat packs".

First it has a very large heat of crystallisation, which means it gives out a significant amount of energy on solidifyinghence the temperature rise to around 50 °C. Secondly, the solution exists as a metastable, supercooled liquid ...

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